A lithium ion battery is referred to also as a lithium secondary battery, and examples thereof include a lithium ion battery that has a liquid-, gel-, or high polymer-type electrolyte and uses high polymers as positive electrode and negative electrode active materials. A lithium ion battery has a configuration including a positive electrode current collecting material, a positive electrode active material layer, an electrolyte layer, a negative electrode active material layer, a negative electrode current collecting material, and an outer packaging body that packages these materials and layers, and a multilayer film is used as a packaging material for forming the outer packaging body.
FIG. 10 is a perspective view of a conventional lithium ion battery, and FIG. 11 is a sectional view of the lithium ion battery taken on line A-A′ in FIG. 10. As shown in FIGS. 10 and 11, a lithium ion battery 101 is configured by hermetically housing a lithium ion battery main body 120 in an outer packaging body 110 constituted at least of a base layer 111, a metal foil layer 112, and a heat-sealable resin layer 113.
The lithium ion battery main body 120 is constituted of a cell (electricity storage portion) including a positive electrode composed of a positive electrode active material and a positive electrode current collector, a negative electrode composed of a negative electrode active material and a negative electrode current collector, and an electrolyte filled between the positive electrode and the negative electrode (none of these is shown in the figures), and a metal terminal 121 that is connected to each of the positive electrode and the negative electrode in the cell and whose tip end protrudes to the outside of the outer packaging body 110.
In the manufacturing process of the lithium ion battery 101, however, there has been the following problem. That is, when the mutually opposed heat-sealable resin layers 113 of the outer packaging body 110 are overlaid on each other, and a peripheral edge portion thereof is heat-sealed so that the lithium ion battery main body 120 is hermetically housed therein, the heat-sealable resin layer 113 might be thinned, due to heat and pressure applied thereto at the time of the heat-sealing, to such an extent that a short circuit occurs between the metal terminal 121 and the metal foil layer 112. Furthermore, after the heat-sealing, if the outer packaging body 110 is bent, a crack might occur in the heat-sealable resin layer 113, through which the electrolyte filled in the outer packaging body 110 permeates from part of the heat-sealable resin layer 113 into the metal foil layer 112, leading to a short circuit, or, even if a short circuit has not yet occurred between the metal terminal 121 and the metal foil layer 112, a minute crack might occur in the heat-sealable resin layer 113, which will possibly lead to a short circuit in the future.
Conventionally, as a solution to this problem, from among completed lithium ion batteries, samples for inspection are drawn at a predetermined frequency, with respect to each of which a predetermined high voltage is applied between the metal terminal 121 and the metal foil layer 112, and among the samples of the lithium ion battery 101, those in which, as a result of the voltage application, an insulation breakdown has occurred in the heat-sealable resin layer 113 are all removed from the production line as defective products having cracks that might lead to a short circuit. This inspecting method, however, has been disadvantageous in that the inspection can be performed only with respect to lithium ion batteries drawn as samples, and in that, due to low accuracy of the inspection, such a minute crack that it would not lead to a short circuit can hardly be identified. This inspecting method has been disadvantageous also in that, as a result of applying a high voltage, it is highly likely that some samples of the lithium ion battery 101 that are in fact not defective products have newly suffered from an insulation breakdown and thus cannot be reused after the inspection.
Furthermore, Patent Document 1 describes a method in which plating is performed to deposit metal on a metal foil layer constituting an outer packaging body, and an insulation-deteriorated area is determined by checking, by visual observation or the like, whether or not a metal deposit is formed thereby. This inspection method described in Patent Document 1, however, requires that a lithium ion battery be soaked in a plating bath and thus has been disadvantageous in that a lithium ion battery that has undergone the inspection cannot be reused irrespective of a result of the inspection.